(1) Field of the Invention
The present invention relates to a nitride electronic device intended for power switching as represented by an inverter used for an electric vehicle or a home electric appliance.
(2) Description of the Related Art
Recently, a nitride semiconductor has been under intense research and development as a material for high-output devices.
A Modulation Doped Field Effect Transistor (MODFET) applying an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterojunction is being studied as an electronic device. The most significant difference between such MODFET and a gallium arsenide (GaAs) MODFET is that the former MODFET can realize a sheet carrier concentration ten times as much as that of the GaAs MODFET without doping, with impurities, an AlGaN layer which is a Schottky layer. The mechanism of carrier generation is that two-dimensional electrons are accumulated in an interface between AlGaN and GaN, which is caused by a polarization generated due to a piezo effect in the AlGaN layer because of the stress between AlGaN and GaN due to the lattice mismatch. Therefore, stress is a very important parameter for the MODFET applying the AlGaN/GaN heterojunction, and thus the relationship between the sheet carrier concentration and the stress between AlGaN and GaN is vigorously being researched. For example, sheet carrier concentration of two-dimensional electron gas is quantitatively calculated based on stress, as described by O. Ambacher et al. in Journal of Applied Physics (1999), Vol. 85, pp. 3222-3233.
As is represented by the example shown in the above-mentioned reference by O. Ambacher et al., the conventional AlGaN/GaN heterojunction is formed by stacking layers in a direction of C-axis. This is because, with such formation of heterojunction, namely, with the formation of an AlGaN/GaN heterojunction having a C-plane, the effect of polarization unique to the nitride semiconductor can be obtained.
In contrast, Japanese Laid-Open Patent Application No. 2001-160656 (hereinafter, referred to as Patent Reference 1) discloses a field effect transistor (FET) in which an A-plane or an structure is used as a main surface of a substrate and a direction of current is made vertical to the C-axis. With such FET, a dispersion of carriers due to dislocation is suppressed so that the FET with excellent electric characteristics can be realized.